spi: orion: Rework GPIO CS handling

- Claim the GPIO from the driver, not via DT bindings or through the
  platform code
- Find an unused HW CS signal because Orion needs to drive one for each
  SPI transaction

The spi-orion.c was the only driver which supported (or cared about) the
CS GPIO, while it wasn't actually requesting it. This change means that
the DT bindings should stop hogging the GPIO CS pins because it's now
being handled by the driver.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>